1. Field of the Invention
The present invention relates to a manufacturing method of a high electron mobility transistor (HEMT) having an AlGaN/GaN hetero structure.
2. Background Art
GaN has a wider bandgap than prior-art Si and GaAs and its breakdown field is higher by one digit. Therefore, GaN HEMT is difficult to be broken even under a high electric field and is suitable for a high-voltage operation. Moreover, GaN has a larger saturated electron speed in a high electric field region than prior-art semiconductor materials. This is advantageous for a high-speed operation of a device, and the effect is particularly marked in a high-frequency device with a short channel in which a high electric field is applied to the vicinity of a gate electrode. For manufacture of this GaN HEMT, a metal organic chemical vapor deposition (MOCVD) method is used in many cases.
A sapphire substrate has been widely used for epitaxial growth of GaN by the MOCVD method, but since its lattice matching performance with GaN is low and thermal expansion coefficient is different, GaN crystallinity could not be improved in the past. After that, a technology of a low-temperature buffer layer has been established, and GaN crystallinity after growth is improved, and a device fabrication technology mainly for blue and white LED applications has achieved a rapid progress. However, since further improvement of crystallinity such as lower dislocation and the like is difficult and low heat conduction of the sapphire itself results in insufficient heat radiation when being integrated into a device and lowered device performance, and there is a persistent demand for alternative substrate materials. SiC with high lattice matching performance and stability at a high temperature is one of candidates, but it has a problem that its high cost makes an increase of a diameter difficult in addition to a problem of a quality of the crystal itself such as a micro pipe despite the recent improvement.
On the other hand, a Si substrate is capable of realizing a sufficiently larger diameter and lower dislocation and is stably available at a low cost. However, in a state in which a GaN layer is accumulated on a low-temperature AlN buffer layer and then, it is returned to a room temperature, in a case of a sapphire substrate, a compression stress acts on the GaN layer and a crack is hardly generated, while in a case of the Si substrate, differences in the lattice matching performance and heat expansion coefficient, a tensile stress results in a crack to be generated easily. In addition, there have been many problems such as melt-back etching related to a reaction between Ga (or GaN) and Si and the like, but technological development such as a multi-layer film buffer layer has progressed recently and has achieved a sufficiently practical level.
In the GaN HEMT, on a substrate of sapphire, SiC or Si, an undoped GaN layer and an undoped AlGaN layer are sequentially formed through a buffer layer for relaxing lattice mismatch. In the vicinity of an interface between the GaN layer and the AlGaN layer, a piezo effect caused by distortion of the AlGaN layer and high-concentration two-dimensional electron gas is induced by spontaneous polarization, whereby a channel portion of the GaN HEMT is generated (see Japanese Patent Laid-Open No. 2001-44126, for example).